Camera-based inspections often require lighting to be optimized for a given application. Optimized lighting for an application can become difficult when parts vary in geometry and/or finish.
Direct Part Marking (DPM) is a technology whereby an item is physically altered by methods such as dot peening, laser etching, molding, and/or embossing to produce two different surface conditions.
Marking can result in the symbol being created by either smooth and rough (or dull and shiny) areas on the substrate or a symbol comprised “dots” below or above the substrate surface. DPM is typically used with matrix symbologies.
As a result, there is often very little or no contrast between the “marked” elements of a symbol and the background (substrate). Contrast can be achieved by using specific types of lighting and signal processing techniques.
A Data Matrix is a type of barcode that relies on print quality grading systems to ensure that the codes are printed well and will be readable in the future. The Association for Automatic Identification and Mobility (AIM) maintains a specification for Data Matrix verification, which is a process of determining the print quality of, for example, a “barcode”, a symbol created by Direct Part Marking, etc. Using the traditional barcode as an example, let's say that you were printing the barcode and the toner on your printer was running low, such that you have a section where the barcode is faded and/or missing some of the bars. You also might have an overzealous courier who makes a black line with a marker through the barcode. These types of damage to the code can make it difficult or impossible to read the barcode. The same principle exists for 3D barcodes. For example, if a laser is marking a fuel injector and the beam passes over a drop of oil, it might not mark the part in that section. The specification mentioned above has a series of tests for issues such as missing parts of the code, faded codes, and angular distorted codes. If you were to put a mark on a mirror surface or a flat black cylinder, if you do not utilize the right light, you might not even see the mark. The following text is the summary of the AIM specification's currently required lighting:
Lighting
Two lighting environments each with sub-options are defined in this document. The lighting environments are denoted in the reported grade using the format defined in ISO/IEC 15415, section 6.2, using the angle specifier with a combination of numbers and letters as defined below.
Note: This does not limit the prerogative of an Application Specification to choose different lighting environments based on application requirements. Alternate lighting environments should include specifiers consistent with the format of those below that can be used for communicating quality requirements and quality grades.
Diffuse Lighting
Diffuse Perpendicular (On-axis/Bright Field) (90)
A flat diffusing material is oriented such that the plane of the material is parallel to the plane of the sensor. The symbol is uniformly illuminated with diffuse light incident at 90 degrees to the plane of the symbol. The angle specifier shall be 90.
Diffuse Off-axis (D)
A diffusely reflecting dome is illuminated from below so that the reflected light falls non-directionally on the part and does not cast defined shadows. This is commonly used for reading curved parts. The angle specifier shall be D.
Directional Lighting
Two orthogonal lighting planes are defined such that both planes are perpendicular to the surface of the image sensor and one plane is parallel to the line formed by a horizontal edge of the image sensor to within +/−5 degrees. Light is directionally aimed at the part such that the line formed by the center of the light source and center of the symbol rests in one of the lighting planes and is at an angle of 30+/−3 degrees from the horizontal plane of the surface of the symbol. The lighting shall illuminate the entire symbol area with uniform energy.
Low Angle, Four Direction (30Q)
Light is aimed at the part from four sides. The angle specifier shall be 30Q.
Low Angle, Two Direction (30T)
Light is aimed at the part from two sides in one of the two lighting planes. The angle specifier shall be 30T.
Low Angle, One Direction (30S)
Light is aimed at the part from any one of the four sides. The angle specifier shall be 30S.
Thus, conventional technology can require a user to manually change the lighting used for each of a number of different verification methods.